Patient‐Specific Mechanisms of Lafora Disease Mutations in the Human Glycogen Phosphatase

Lafora disease (LD) is a fatal neurodegenerative epilepsy caused by defects in glycogen metabolism. Classically, LD manifests in adolescence with progressive myoclonus epilepsy, rapid neurodegeneration and death 10 years after onset. Insoluble inclusions of abnormal glycogen known as Lafora bodies (LBs) are found in the brain and other tissues of Lafora patients. Genetic ablation of glycogen synthesis eliminates LBs and rescues LD in mice, indicating LBs are the pathological cause of LD. 50% of LD cases are caused by recessive mutations in the Epilepsy Progressive Myoclonus 2A ( EPM2A ) gene encoding laforin, the glycogen phosphatase. Laforin is a bimodular protein containing a carbohydrate binding module (CBM) and a dual specificity phosphatase domain. In the absence of laforin, glycogen becomes hyperphosphorylated and abberantly branched, leading to LB formation. >90 distinct missense, nonsense, and frameshift mutations have been described in LD patients. We previously determined the crystal structure of laforin and defined the structural basis of its function. The objective of this study is to define the molecular defect(s) of >30 pathogenic mutations from LD patients and determine why some are linked to milder phenotypes. Using the laforin crystal structure and a variety of cellular, biochemical, and biophysical tools, we discovered a hierarchy of mutations that differentially affect aspects of laforin function. Specifically, we discovered that a large group of pathogenic mutations cluster in the CBM, leading to destabilization, domain decoupling, and reduced protein‐protein interactions. Further, a mutation associated with an extremely mild case of LD impairs dimerization and preferential binding to abnormal glucan substrates. Our study reveals the importance of the CBM for both stability and glucan binding, defines the atomic‐level effects of patient mutations, and provides a biochemical explanation for late‐onset phenotypes. Our work not only provides a framework for defining novel LD mutations and understanding patient‐specific disease progression, but also deepens our understanding of the enzymatic regulation of glycogen. Support or Funding Information This work was supported by NIH Grants R01 NS070899 (M.S.G.), P01 NS097197 (M.S.G.), and F31 NS093892 (M.K.B.); NSF Grant IIA‐1355438 (M.S.G.); Mizutani Foundation for Glycoscience Award (M.S.G.). This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .